1. Field of the Invention
This invention relates generally to a method of making a linearity standard magnetic tape and, in paticular, to a method of producing a linearity standard magnetic tape suitable for use for adjustment and inspection of a video tape recorder (VTR) of the type in which tracking pilot signals of low frequency and a video signal are superimposed on each other and then sequentially supplied to, for example, two video heads having gaps of different azimuth angles arranged in a rotary head assembly so as to record the video signal in overlapping tracks, and in which, upon playback, the tracking pilot signals are used to achieve correct tracking by the video heads.
2. Description of the Prior Art
During the manufacturing of a VTR having a rotary magnetic head assembly which includes two magnetic video heads with different azimuth angle gaps located apart from each other by a predetermined angular distance therebetween, and in which a video signal is normally supplied to the two magnetic video heads so as to record the video signal in overlapping tracks, a linearity standard magnetic tape is used. Such tape has recorded tracks of a signal of a predetermined frequency used for adjustment and inspection, and the tracks are recorded by using only one of the two magnetic video heads which have different in azimuth angle gaps. Generally, the record track on this standard tape is required to have excellent linearity, and therefore, during the manufacturing, it is necessary to inspect the linearity of the record track on the standard magnetic tape and to maintain a predetermined linearity thereof at all times.
For inspecting the linearity of the record track on the linearity standard magnetic tape, a suspension of carbonyl iron (Fe.sub.3 O.sub.4) and a diluent such as a volatile liquid of "Freon" is coated on a predetermined area of this linearity standard magnetic tape and then a magentized pattern, in particular record tracks on the magnetic tape are developed or made visible as shown in FIG. 1. Then, as also may be seen from FIG. 1, the linearity is inspected by a so-called transverse method, in which the track distance between adjacent points along a one-dot chain line is measured. In this case, since the inspection signal is recorded on the linearity standard magnetic tape by using only one of the two magnetic video hands, there exist nonmagnetized portions between adjacent record tracks. Thus, the linearity of the linearity standard magnetic tape can be measured by this transverse method.
For 8 mm video VTRs, according to the standard specifications of "The 8 mm Video Standardization Conference" released on Mar. 28, 1983 (Weekly Television Digest, Apr. 4, 1983, Vol. 23:14), four pilot signals of low frequency, for example, f.sub.1 =about 102 KHz, f.sub.2 =about 116 KHz, f.sub.3 =about 160 KHz and f.sub.4 =about 146 KHz, are sequentially in superimposed, in a repeating cycle, on a video signal recorded on a magnetic tape in successive tracks by two magnetic video heads having different azimuth angle gaps arranged in a rotary magnetic head assembly. Upon playback, an ATF (automatic track finding) system is employed in which these pilot signals are used to control the tracking by the video heads.
If the above described linearity standard magnetic tape is used for such ATF-type VTRs and the recording is made by using only one of the two magnetic video heads as in the prior art, only one set of the pilot signals f.sub.1 and f.sub.3 (or f.sub.2 and f.sub.4) is recorded. As a result, if such linearity standard magnetic tape is played back, there results the disadvantage that the tracking on the VTR can not be achieved. Further, if the two magnetic video heads are used to sequentially record the pilot signals f.sub.1, f.sub.2, f.sub.3 and f.sub.4 repeatedly together with a predetermined measuring signal or a bias signal of predetermined high frequency, the magnetized tracks can not be visibly distinguished from each other even if the afore-noted coating is used, and so it is impossible to measure the linearity of the magnetized tracks during manufacturing of the linearity standard magnetic tape.